Using a conventional clutch mechanism, which disconnects the input shaft of the gearbox from the combustion engine during a shifting process in the gearbox, entails disadvantages, such as heating of the clutch mechanism's discs, which results in an increased fuel consumption and wear of the clutch discs. There are also large losses as a result, in particular when the vehicle is started. A conventional clutch mechanism is also relatively heavy and costly. It also occupies a relatively large space in the vehicle. Friction losses also arise at the use of a hydraulic converter/torque converter commonly used in automatic transmission. By ensuring that the vehicle has a drive system in which the output shaft of the combustion engine, the rotor of the electrical machine and the input shaft of the gearbox are connected with a planetary gear, the conventional clutch mechanism and disadvantages associated therewith may be avoided. A vehicle with a drive system of this type constitutes prior art as set out in EP 1 319 546 and SE 1051384-4.
Although this drive system, especially the one described in SE 1051384-4, functions well and has a range of advantageous features, efforts are constantly made to improve such a drive system with respect to its behavior and function in certain operating situations.
The present invention thus also relates to a drive system for a vehicle as set out above.
Conventional drive systems are adapted so that they only offer a small number of operating modes, which are adapted for the main loads at the operation of the vehicle. One disadvantage of conventional drive systems is that the operating modes result in a relatively fuel-intensive operation of the vehicle and a limited torque range at a low load.
U.S. Pat. No. 6,354,974 shows a drive system for a hybrid vehicle. The hybrid vehicle comprises a combustion engine and an electrical machine, which is arranged on the combustion engine's output shaft. The objective here is to create a compact drive unit, which does not need to use a conventional clutch mechanism. The conventional clutch mechanism has here been replaced by a planetary gear and three friction clutches. With the help of the friction clutches, different operating modes may be created in the vehicle. The use of friction clutches results in energy losses.